1. Field of the Invention
The present invention relates to an improvement in a non-aqueous electrolyte secondary battery and a method for producing the secondary battery. In particular, the present invention relates to a non-aqueous electrolyte secondary battery that can achieve high reliability in a battery having high capacity and a method for producing the secondary battery.
2. Description of Related Art
In recent years, mobile information terminals such as cellular phones, notebook computers, and personal digital assistants (PDAs) have been rapidly decreasing in size and weight. With such a trend, a further increase in the capacity of batteries serving as driving power sources of such mobile information terminals has been demanded. To meet such a demand, a non-aqueous electrolyte secondary battery that uses an alloy which can occlude and release lithium ions, a carbon material, or the like as a negative electrode active material and uses lithium transition metal complex oxide as a positive electrode active material has been receiving attention as a battery having high energy density.
An increase in the capacity of existing non-aqueous electrolyte secondary batteries has been achieved by decreasing the thickness of a component such as a battery can, a separator, or a current collector (aluminum foil or copper foil) that is unrelated to capacity or by achieving a high packing density of an active material (by improving the packing density of an electrode). However, even if these means for increasing the capacity are employed, the capacity of non-aqueous electrolyte secondary batteries cannot be increased markedly. It is considered that by increasing the charge cut-off voltage, the capacity and energy density are increased. However, in the case where the charge cut-off voltage is increased, a positive electrode active material is degraded and an electrolyte is decomposed by oxidation, which poses a problem in that the battery characteristics are degraded.
In view of the foregoing, the surface treatment and the stabilization of a structure of a positive electrode active material have been actively researched. For example, there have been the following proposals.
(1) In the case where the charge cut-off voltage is increased, the stabilization of a structure of a positive electrode active material is achieved by incorporating a Zr element into a lithium transition metal oxide that has a layered structure and contains lithium and cobalt (see Japanese Patent No. 4307962 (Patent Document 1)).
(2) A technique in which at least part of a surface of a positive electrode active material is coated with a surface treatment layer composed of a phosphate compound represented by MPOk (M is at least one trivalent element and k is an integer of 2 to 4). According to this technique, by suppressing the reaction between an electrolyte and a positive electrode, the cycle characteristics are improved without decreasing the initial efficiency (see Japanese Published Unexamined Patent Application No. 2005-243301 (Patent Document 2)).
(3) A battery having good high-temperature swelling characteristics (that is, not swell even at high temperature) by using a positive electrode active material obtained as follows. An active material precursor is added dropwise to a coating solution obtained by mixing a phosphorus compound having a double bond such as (NH4)2HPO4, a compound containing Al such as Al(NO3)3.9H2O, and water. A lithium source is added thereto and heat treatment is performed to obtain the positive electrode active material (see Japanese Published Unexamined Patent Application No. 2005-166656 (Patent Document 3)).
However, in the technique disclosed in Patent Document 1, when the charge cut-off voltage is increased, the structure of a positive electrode active material can be stabilized to some degree, but the degree of the stabilization is insufficient. Thus, the battery capacity is significantly reduced when a battery is stored at high temperature. The techniques disclosed in Patent Documents 2 and 3 provide a structure in which a positive electrode active material is coated with a compound of aluminum or lanthanum. In the case where the charge cut-off voltage is 4.2 V, the effects achieved by such a structure are produced to some extent. However, in the case where the charge cut-off voltage is further increased (e.g., the charge cut-off voltage is increased to 4.4 V), the above-described effects are not sufficiently produced.